The use of borosilicate glass for culinary ware is well-known to the art. Such glassware has been marketed under the trademark PYREX.RTM. for over 50 years. Most generally, the glassware has been marketed in the clear or "water white" state and decorating thereof accomplished through staining or the application of surface decorations thereto. Staining is a relatively slow, expensive process which is customarily employed to color small areas only of a glass article. Surface decorating of glass aticles has been widespread, the designs of such being essentially limitless. Staining of glass, commonly involving an ion exhange reaction in the glass surface, provides a reasonably permanent coloration to the glass since the ion exchange reaction results in ions penetrating and diffusing into the surface of the glass. In contrast, as the term suggests, surface decorations are simply that and, consequently, are subject to removal via chemical and mechanical abuse. This problem has been exacerbated through the increasing use of very hot water and strong detergents, such as are employed in home dishwashers.
The use of inorganic colorants to tint glass, i.e., to impart an integral color thereto, is well-recognized in the art. Colured Glasses, Woldemar A. Weyl, Dawson's of Pall Mall, London, 1959 is a standard text providing a survey of many ions which have that capability and suggesting mechanisms giving rise to the colors developed. An apparent advantage inherent in the use of such colorants is that the color produced becomes an integral part of the glass structure and, as such, is not subject to chemical attack and mechanical abuse.
Extensive consumer sampling had indicated that a clear glass having a gray-brown tint would be highly desirable in culinary ware. Target samples were prepared by spraying clear tinted lacquers of various colors and shades on borosilicate glass bodies. Glass compositions were then devised which visually matched the color of the target samples. Transmittance measurements were made on each glass. Consumer interviewing, utilizing articles formed from each glass composition in shapes conventional for culinary ware, determined that a clear glass having a light gray-brown color approximating the transmittance curve constituting appended FIGS. 6, 8, and 9 would be the most appealing.